There are not actually 2 versions of Gath, rather then 2 versions of his weapons. At the point where you are about to screw up the plot by killing some vital char, he will come with his meano sword and whoop you bad, as a lesson.

Lateron, when you are past that critical spot (and on Shadow Thieves side) he will come and have the 'usual' weapon. Being the curious person, I tried that sword. For a very short time, since it was super powerful and thus made the game super boring.